When using contactless chip cards and the like, such as “contactless tags”, use is frequently made of “ASK modulation”. This is understood to be a radio-frequency signal which alternates between a first and a second level with data available in digital form and thus modulates the radio-frequency signal.
In the same way as a distinction is made for digital data between “Yes” and “No” or “1” or “0” or “high” and “low”, a distinction is made between a high amplitude and a low amplitude. In this context, the two modulation types ASK 100 and ASK 10 are usual at the moment, where ASK 100 signifies a level difference of 100% and ASK 10 signifies a level difference of 10%. Other differences are also possible, however, and the invention described below is not restricted to these two usual modulation types.
The problem of ASK modulation can be seen in that a change in the distance between the transmitter and the receiver of the signal modulated in this manner while the amplitude of the transmitted signal remains the same also results in a change in the received amplitude at the receiver end when the distance is altered. The same applies if differences arise in the interspace between the transmitter and the receiver.
As a further complication, when using signals which always return to “0”, i.e. the signal returns to “0” between two binary “1 ”, and signals which have no provision for this, “0” and “1” sequences of different lengths are modulated and transmitted.